The present invention relates to self-assembled organic ligand monolayers on the surface of metal oxide or silicon oxide substrates. In particular, the present invention relates to monolayers in which transition metal atoms selected from Group IVB, Group VB or Group VIB of the Periodic Chart are covalently bonded to the surface oxygen of the substrate, wherein each transition metal atom is further covalently bonded to one or more organic ligands of the monolayer, thereby bonding the organic monolayer to the surface of the substrate. In addition, the present invention relates to methods of forming such self-assembled organic ligand monolayers, by providing a metal oxide or silicon oxide substrate having a surface layer of alkoxides of transition metals selected from Group IVB, Group VB, or Group VIB of the Periodic Chart covalently bonded thereto, wherein the alkoxides are bonded at the transition metals to the surface oxygens of the substrate, and then reacting the transition metal alkoxide surface layer with an organic compound capable of reacting with transition metal alkoxides to form a covalent bond between the transition metal and a ligand of the orgasmic compound, so that organic ligands of the transition metals are formed as a self-assembled organic ligand monolayer on the surface of the substrate, covalently bonded at the transition metals to the surface oxygens of the substrate.
When assembled, organic molecular film monolayers are at the center of much current research. There are applications for self-assembled monolayer (SAM) films in fields such as microelectronic packaging, lubrication, catalysis and electrochemical applications.
To date, SAM's are formed by the absorption and spontaneous organization of amphiphilic molecules on a metal or metal oxide substrate. Typically, SAM's are formed from a solution of the amphiphilic material in which the substrate surface is immersed over a period of time during which a film is formed. When the substrate is removed from solution, the absorbed film is retained on the substrate surface.
The characterization of SAM films has revealed that there is a high degree of order, but the films have not attained the same degree of structure that Langmuir-Blodgett (LB) films exhibit. LB film structure is characterized by tightly packed molecules oriented at a consistent bond angle to the surface normal. This is attributable to the formation of LB films on smooth, even surfaces that allow the molecules to pack in the closest arrangements allowed by the size of the head/tail groups. The film coverage of the surface is even and uniform.
In the formation of SAM's, the surfaces are typically rough and uneven. This reduces the density of coverage and makes the film uneven. The uneven nature of the film increases the possibility for defect and breaches in the film integrity. Although SAM monolayers have not achieved the same degree of order, they offer a distinct advantage over LB monolayers in the simplicity of their formation.
Laibinis et al., Science, 245, 845 (1989) disclosed that a substrate surface composed of alumna and gold regions, when exposed to a common solution containing alkanethiols and organic carboxylic acids forms two homogenous SAM's, an alkanethiol-gold SAM and an organic carboxylic acid-alumna SAM, independently and simultaneously. Alkanethiols are also known to form SAM's on silver. The high affinity between the alkanethiol and the gold or silver surface allows a great variety of functional groups to exist at the other end of the alkane moiety. However, no practical use has been found for the alkanethiol/gold system. Protection for gold is redundant because gold is not a very reactive metal. The system could be used an insulator in microcircuit packaging, but gold is very expensive. The use of alkanethiols and other metals or metal oxides has not proven to be practical because thiols do not absorb onto many metal oxide surfaces.
The problem of stability is preventing the widespread success of organic carboxylic acid SAM's. The absorption of organic carboxylic acids on native metal oxides has proven to be a very weak interaction. While the SAM procedure is simple, losses in film stability and reductions in surface coverage have been experienced.
In attempting to reproduce carboxylic acid SAM's made on aluminum oxides reported in the literature, the organic carboxylic acids would not absorb irreversibly onto the aluminum oxide surface. There exists the need for organic SAM's of improved stability on native metal oxides for application in areas such as microelectronic packaging coating and biological implant coatings.